The present invention is related generally to methods for bonding semiconductor devices to substrates, and in particular, to a method for low temperature solder bonding of temperature sensitive devices such as light-emitting diode (LED) assemblies to a substrate containing a heat sink, without exposing the entire semiconductor device or the adjacent substrate to damaging heat.
Light-emitting diodes, or LED's, are semiconductor light emitting components that are increasingly replacing incandescent bulbs in many lighting application. As shown in FIG. 1, a prior art typical LED assembly consists of an LED chip (not shown) encased in a protective package 31 (LED package). Two or more connection leads 34 with feet 36 extend from the sides of the package 31. A thermally coupling slug 32, commonly formed from copper, is disposed at the base of the LED package 31 and is coated with a wetting layer 33 to facilitate bonding to a substrate which incorporates a heat sink such as a metal core printed circuit board (MCPCB) (not shown in FIG. 1). A lens 35 may be attached to the top of the LED package 31 for directing emitted light. An exemplary LED package 31 may have dimensions of approximately 5 mm width×5 mm length by 2 mm height, without the lens 35.
For economic fabrication the LED package 31, the lens 35, and the MCPCB substrate desirably comprise polymer or plastic materials. The MCPCB substrate is typically a printed circuit board that includes a metal core within one or more layers of dielectric material. Copper traces and pads are disposed outside the dielectric material, and a solder mask is disposed surrounding the copper pads and covering the traces. FIG. 2 shows a typical pad pattern to match the connection leads 34 and slug 32 of the LED package 31 shown in FIG. 1. For operation of the LED package 31, the slug 32 is bonded to pad 55b while the two leads 34 are each bonded to an associated pad 55a. 
In one conventional bonding process, the various bonds between the LED package 31 and the MCPCB pads 55a, 55b are formed by a high temperature solder reflow process. However, polymer lenses 35 may be damaged by the solder reflow process due to exposure to the high temperature reached during the solder reflow, and particularly to the high temperatures used with lead-free solder reflow procedures. Similarly, where large heat sinks are needed, plastic packaging and the LED semiconductor electronics may be damaged by reflow heating because the thermal mass of the heat sinks necessitates a lengthy solder reflow cycle at an elevated temperature. If the solder reflow cycle is not long enough, poor bonding may result due to inadequate melting of the solder. In addition, the high temperature of reflow may adversely affect the brightness of the LED or the lifetime of the device.
Alternatively, the connection leads 34 may be soldered individually using laser or hot bar (thermode) soldering and the center slug 32 may be attached with a thermally conductive adhesive. An electrical connection between the center slug 32 and the pad 55b is not needed, but since the center slug 32 serves to convey heat from the LED package 31 to the metal-core PCB, there must be low thermal resistance between the center slug 32 and the MCPCB. Thermally conductive adhesives have reasonable but limited thermal conductivity, and suffer from inconsistent viscosity during application which can lead to varying bond line thicknesses. If the thickness of the bond line between the center slug 32 and the pad 55b is too great, the thermal resistance of the bond increases. In extreme situations, the thickness of the bond at the center slug 32 can be so great that the feet 36 of the connection leads 34 fail to contact the pads 55a, making laser soldering difficult or impossible. Consequently, conventionally fabricated LED assemblies often fall short of their potential thermal, electronic and optical performance potential.
The same problems may occur in other electronic components and semiconductor packages with integrated thermal management pads or slugs that must be bonded to an external heat sink, and which are sensitive to high bonding temperatures. Such components and packages include but are not limited to electrolytic capacitors, power amplifiers, and photovoltaic devices. Many components that were designed for lead-tin eutectic solders are damaged at the higher reflow temperatures required for the use of lead-free solders.
Accordingly, it would be advantageous to provide a uniform and reliable method for bonding electronic packages, such as semiconductor devices and LEDs which are sensitive to extreme bonding temperatures, to a supporting substrate. It would be further advantageous to provide such a bonding method which does not generate excessive heat outside of the bonding region, and which results in bonds having a uniform thickness and strength, and which are capable of conducting electrical current and thermal energy.